A number of medical procedures involve or can be supplemented with the placement of an endoluminal prostheses, commonly referred to as a stent, that can be implanted in a lumen, such as a blood vessel or other natural pathway of a patient's body. Such stents typically define a generally tubular configuration, and are expandable from a relatively small diameter (low profile) to an enlarged diameter. In its low profile configuration, the stent can be advanced endoluminally, by a delivery device, through the body lumen to the site where the stent is to be placed. The stent then can be expanded to a larger diameter to firmly engage the inner wall of the body lumen. The delivery device then is removed, leaving the implanted stent in place. In that manner, the stent may serve to maintain open a blood vessel or other natural duct, the functioning of which had become impaired as a result of a pathological or traumatic occurrence.
Among the medical procedures in which stents have had increasing use is in connection with percutaneous transluminal angioplasty (PTA), and particularly percutaneous transluminal coronary angioplasty (PTCA). PTA and PTCA involve the insertion and manipulation of a dilating catheter through the patient's arteries to place the dilatation balloon of the catheter within an obstructed portion (stenosis) of a blood vessel. The balloon then is expanded forcibly within the obstruction to dilate that portion of the blood vessel thereby to restore blood flow through the blood vessel. Among the more significant complications that may result from such angioplasty is that in a significant number of cases, the dilated site again becomes obstructed. By placing a stent within the blood vessel at the treated site, the tendency for such restenosis may be reduced. Accordingly, a number of stents have been proposed and developed.
One such stent is disclosed in U.S. Pat. No. 4,800,882 (Gianturco) in which a tubular stent is formed from a single, continuous metal wire that is bent into a planar serpentine configuration extending longitudinally along what will become the axis of the stent. The transversely extending loops of the serpentine wire then are deformably wrapped circumferentially about the axis to define a generally cylindrical surface and the tubular configuration. The stent so formed may be considered to define a plurality of circumferentially curved C-shaped segments, each connected to its adjacent segment at a reversing bend, so that each curved section extends in the opposite circumferential direction than its adjacent curved sections. Consequently, none of the C-shaped curved segments defines a closed circumferential loop. The endoprosthesis then is mounted about the balloon of a delivery catheter and the stent then is crimped about the deflated balloon to its low profile configuration. With the stent so mounted on the balloon, the catheter and stent are advanced through the patient's vasculature to the stenosis where the balloon is inflated to dilate and expand the stent radially and plastically to the dimensions intended. The C-shaped configuration of the curved segments of the stent necessarily and undesirably limits the resistance of the stent to radial compression, as can occur within an artery after an angioplasty has been performed. Increasing the radial resistance to contraction by increasing the thickness of the wire from which the stent is made is an unsatisfactory solution because that necessarily will require an increase in the thickness of the stent which will narrow the cross-section of the lumen. Moreover, the discontinuities in the lateral surface defined by the stent may tend to disturb the fluid dynamics of blood flowing through the blood vessel that, in turn, could induce turbulence with resulting generation of emboli, thrombi and other serious complications.
Also among the difficulties with the above-described device is that it presents little protection for the balloon of the delivery catheter when the catheter is advanced through the patient's vasculature to the deployment site. As the stent and delivery catheter are advanced, the relatively open configuration of the stent exposes the balloon to the walls of the blood vessel. Those walls may have rigid encrustations of arteriosclerotic plaque that can be irregular and sharp. Consequently, when the stent is advanced past plaque formations, the balloon may be punctured or damaged. That may result in bursting of the balloon when it is subsequently inflated, presenting a danger to the patient.
Also among the difficulties presented with the above-described stent is that its single wire construction does not readily lend itself to precise matching to the vascular anatomy or pathological situation of the specific patient in whom the stent is to be placed. The construction is adapted, as a practical matter, only to being manufactured in standard lengths. When a standard length of stent does not ideally match the patient's anatomy, the physician must choose among those standard lengths in an effort to select one or more. That is, at best, a compromise.
Still another disadvantage of the stent design described above, as well as other stent designs that have a fixed configuration (see, for example, the stent disclosed in EP 335,341) is that when the endoprosthesis must be positioned near a branch in the blood vessels, the implantation of the endoprosthesis in one of the branches may obstruct flow into the other branch.
It also is important that the location and position of the endoprosthesis be determined during implantation as well as at a later time. The Gianturco stent described above, being formed from a single, slender wire may be difficult, if not impossible, to visualize under fluoroscopy or X-ray.
It is among the general objects of the invention to provide an improved endovascular stent that overcomes the above disadvantages.